Themes

Built-in themes

Hadar includes three themes:

Name	Appearance	Title / body size	Margin / gap
`minimal`	White background, dark text, blue accent	44 / 26	64 / 24
`dark`	Midnight background, light text, blue accent	44 / 26	64 / 24
`warm`	Paper background, brown text, warm accent	46 / 27	68 / 26

Set a theme in the deck’s initial YAML front matter:

---
theme: dark
---
# Quarterly report

Without a theme, Hadar uses minimal. The Ruby API can also select one:

theme = Hadar::Theme.builtin("dark")
deck = Hadar::Deck.open("slides.md", theme: theme)

An explicit theme: argument takes precedence over front matter.

Custom themes

Save a JSONC file, for example presentation.jsonc. JSONC accepts comments:

{
  // Unspecified values keep Hadar's defaults.
  "name": "Ocean",
  "colors": {
    "background": "#102638",
    "text": "#f4f8fc",
    "muted": "#b7c9da",
    "accent": "#79b8ff"
  },
  "font": {
    "family": "sans-serif",
    "title_size": 44,
    "body_size": 26
  },
  "spacing": {
    "margin": 64,
    "gap": 24
  }
}

Load it through the API:

theme = Hadar::Theme.load("presentation.jsonc")
deck = Hadar::Deck.open("slides.md", theme: theme)

An opened deck can also use theme: presentation.jsonc in front matter; the path resolves from the deck’s directory. A parsed deck resolves it from the working directory. If the named custom file does not exist, theme selection falls back to minimal; use Theme.load explicitly when a missing theme should raise an error.

Theme validation

Theme files must be JSON objects no larger than 256 KiB. Unknown keys are rejected. Omitted sections and values inherit the minimal defaults.

name must be a nonempty string.
Colors use six- or eight-digit hex strings: #RRGGBB or #RRGGBBAA.
font.family must be a nonempty string naming an available font family.
Title and body sizes must be finite numbers from 8 to 160.
Margin and gap must be finite numbers from 0 to 512.

Make sure the fonts installed on the rendering machine cover your text. PDF export requires a TrueType-outline font with every visible character; its explicit font: argument selects the embedded font.

On this page

Built-in themes

Custom themes

Theme validation